1. Field of the Invention
The present invention relates generally to automated article handling and, more particularly, is concerned with a gripper mechanism on a robot manipulator arm and the method of operation thereof for removing individual nuclear fuel pellet samples from different depths in a stack of pellets in a sintering boat.
2. Description of the Prior Art
Manufacture of nuclear fuel in pellet form conventionally involves blending radioactive powder material to the desired chemical composition and then fabricating it into pellets. The properly blended powder is made into pellets by first forming it into slugs, then granulating the slugs and mixing a lubricant with the granulates, and lastly pressing the lubricated granulates into green pellets. The green pellets are compactly stacked in layers in receptacles called furnace boats and fed into a sintering furnace where high temperatures sinter the pellets in a hydrogen atmosphere to achieve the required density and microstructure.
However, it is necessary to select representative pellets from the furnace boats immediately after the sintering operation. It is currently required that sample pellets representing top, middle and bottom layers be taken at random. The sampled pellets are examined to determine density and other characteristics prior to releasing the product for further processing.
Current practice is to select the samples manually; however, sintered pellets are hard, relatively heavy, ceramic cylinders with sharp edges. When confined to a container, such as a furnace boat, the pellets tend to form a compact array that is difficult to penetrate. Obtaining samples manually from anywhere but the surface layer is physically difficult, requiring the operator to thrust or "worm" his or her fingers down through the massed pellets.
Also, steps are underway to automate pellet manufacturing lines, such as generally described in the application cross-referenced above, which will make manual access to internal pellet samples even more difficult. For example, automated boat loaders are being implemented to achieve a greater compaction of pellets within the boat by arranging the pellets in interlocking vertical stacks. Further, larger and deeper boats are being used. Conventional boat designs accommodate approximately 1000 pellets, whereas the new boats used in automated operations hold approximately 6000 pellets. Accordingly, a greater number of samples will be required from each boat, but the difficulty in obtaining manual access to the lower layers will be compounded.
Mechanisms for automated handling of articles of various sizes and shapes are known in the prior art. Representative of the prior art are the mechanisms disclosed in U.S. Pat. Nos. to Panissidi (3,583,751), Mack (4,211,123), O'Neil (4,234,223), Birk et al (4,266,905), Thomson et al (4,273,506), Wood (4,348,044) and Higgins (4,421,451). While many of these mechanisms achieve their objectives under the range of operating conditions for which they were designed, most would seem to be unsuitable in terms of the versatility of their operating capabilities for sampling nuclear fuel pellets from a compact layered stack thereof contained in a boat.
Consequently, a need still exists for a suitable automated mechanism and technique to sample nuclear fuel pellets from layers at different depths within the furnace boat in a consistent, repeatable way without damaging the pellets contained therein.